@Sklivvz unfortunately, it looks like that link only contains anedoctal stories like DVK's one in this post...
– o0'.Dec 28 '12 at 16:52

@daniel.selacek Unfortunately, looking at those contradictory studies is all we have to go by. There is almost definitely no causality. As far as helping you make an informed decision, my opinion is that you can't just look at statistics. Depending on what nation you're in, gun ownership can be considered a civil right. When the question of rights comes up (even if they're minority rights), you have to set a very high bar to protect them. My opinion is that significant causality is that bar. But my answer below tries to give a view of the statistics none the less.
– Javid JamaeDec 28 '12 at 17:03

@Lohoris, anecdotal stories are not acceptable on skeptics. Which answers are anecdotal?
– SklivvzDec 28 '12 at 18:08

There is literally 0 correlation between how many people own firearms,
and the homicide rate with firearms, or overall homicide rate as below
graphs and correlation coefficients illustrate.

His graphs are shown here:

In yet another scatterplot analysis of correlation between gun ownership and murder rate by country (again, using Wikipedia as the source), the author find a negative correlation between gun ownership and rate of homicide. He also makes a comment that these statistics aren't very helpful in the debate. From the article:

If all countries are included in the plot, there is a negative
correlation between gun ownership and rate of homicide. Only when the
plot is limited to OECD countries (with the outliers of US and Mexico
included), does a very small positive correlation appear.

...

Because of the many uncontrolled variables involved in comparisons
between countries, I don’t believe such statistics can help the case
for or against gun control. I’m posting them here to counter the
claims by advocates of increased governmental gun control that the
statistics legitimise their preference.

The non‐correlation between gun ownership and murder is reinforced by
examination of statistics from larger numbers of nations across the
developed world. Comparison of “homicide and suicide mortality data
for thirty‐six nations (including the United States) for the period
1990–1995” to gun ownership levels showed “no significant (at the 5%
level) association between gun ownership levels and the total homicide
rate.” Consistent with this is a later European study of data from 21
nations in which “no significant correlations [of gun ownership
levels] with total suicide or homicide rates were found.”

The authors then go on to say that they found a negative correlation between gun ownership and violent crime, meaning that populations with higher gun ownership had fewer incidents of violent crime. From the article:

The same pattern appears when comparisons of violence to gun ownership
are made within nations. Indeed, “data on firearms ownership by
constabulary area in England,” like data from the United States, show
“a negative correlation,” that is, “where firearms are most dense
violent crime rates are lowest, and where guns are least dense violent
crime rates are highest.”

Another book entitled More Guns, Less Crime, written by John Lott, studies crime data for every county in the United States during 29 years from 1977 to 2005 and finds no correlation between gun ownership and murder and concludes that allowing concealed handguns decreases violent crime rates.

The wikipedia page (linked to above) for the book has a slew of papers that argue for and against his conclusions.

There are several sites that display or visualize data, but whose authors don't make any claims or assessments on correlation or causality whatsoever. The Guardian published data in an article entitled Gun homicies and gun ownership listed by country that can be downloaded an analyzed. They also have an interactive heat map that displays the following data:

Average firearms per 100 people

Homicide by firearm rate per 100,000 population

% of homicides by firearm

This is a screenshot of the heatmap:

I took a (very) shallow dive into the data, looking at a 3-4 countries from each continent and there doesn't seem to be much correlation between ownership rates and murder rates. One commenter on the article makes an interesting point that we can't use the univariate solution of Gun Control to solve a multivariate problem of murder. He says:

If the rate of gun ownership is a determinant of the gun murder rate,
how does one explain the gun ownership rates in Mexico (15 guns /100
and 9.97 murders/100k) and Jamaica (8.1 guns/100 and 39.4 murders
/100K) and the very high gun murder rates? Guatemala has a gun
ownership rate of 13.1/100 and a murder rate of 34/100k.

Although our study cannot determine causation, we found that in areas
where household firearm ownership rates were higher, a
disproportionately large number of people died from homicide.

FactCheck.org reviewed studies in 2008 and posted Violent Crimes and Handgun Ownership. They looked at a study by David Hemenway (director of the Harvard Injury Control Research Center and the Harvard Youth Violence Prevention Center), who reviewed commonly cited research from peer-reviewed journals. From the FactCheck.org article:

It found that studies of the United States or U.S. cities, states and
regions "generally find a statistically significant gun
prevalence-homicide association." The report said that the evidence
from such "U.S. cross-sectional studies is quite consistent … where
there are higher levels of gun prevalence, homicide rates are
substantially higher, primarily due to higher firearm homicide rates."

With regard to a causal relationship, FactCheck.org says:

That’s not to say that taking the guns away from such areas would
necessarily lead to a reduction in the murder rate. Such studies have
shown a statistically significant relationship between guns and murder
but not a causal one.

Disclaimer: I haven't personally reviewed the original sources or the analysis behind the sources I'm referencing. I'm not making an argument, one way or the other, as to whether or not I agree with the findings in these articles. I posted these because I think the format and nature of the findings in are specifically relevant to the original question.

Disclaimer 2: I listed many more articles that show little to no correlation, because it was more difficult to find ones that demonstrated a significant correlation. I am happy to add references to more articles that show positive correlation (or even causation) if you post them in the comments.

+1 - As I noted on DVK's answer below, correlation does not imply causation so we have to be careful there. The question is seeking information on causation, but I think you did a good job of laying out the data that is available without implying more than what is appropriate.
– Michael KingsmillDec 27 '12 at 18:00

1

Now for the hard part. Ignore "gun control", and "guns per capita", and try looking at "percentage of households with a hand gun", or even just "percentage of households with a gun". Damn near impossible to find, because no gun registration. Also, see that cluster of "low brady score, high and low homicide rates"? That's two groups. One is "low brady because we want guns no matter what", the other is "low brady because we haven't had a problem"
– Roger WillcocksMay 18 at 7:14

Why did you choose the heatmap which shows "average firearms by 100 people"? Seems to me that the second option "homicide by firearm rate per 100,000" would be more informative for the question.
– ErwanAug 5 at 23:13

In the top charts, is there a datapoint (i.e. state) that records a zero firearms murder rate? Or is it simple low resolution of a low number?
– JontiaAug 7 at 15:57

It appears that there have not been many (or any) peer reviewed studies that show a causal relationship between gun ownership and violent crime. There have been studies that show a statistically significant relationship, but causality as you know is a much higher burden to prove. There is a good discussion of the entire topic in scientific terms at this link.

In summation, it appears that higher gun ownership does imply higher rates of violent crime, but causation cannot be shown in the data available:

That’s not to say that taking the guns away from such areas would
necessarily lead to a reduction in the murder rate. Such studies have
shown a statistically significant relationship between guns and murder
but not a causal one.

You can easily choose which city to examine to make a particular point. For example, if you take New Orleans high crime rates and lack of gun control laws, you can make a case for lack of gun control leading to high crime rates, as this article does, but on the other hand, if you look at the same data points in Chicago, you could make the same argument in reverse, as this article does.

A legitimately objective researcher would find the evidence contradictory. There appears to be more to crime rate than gun control, or the lack thereof.

While statistics can actually show causation, this is only possible if all other variables are controlled. Since most studies only look at existing data, only correlation can generally be demonstrated.

As you see, the answers and comments here also contradict each other. The statistics can be used and misused, it depends who publish them.

Take that article: http://www.landonswan.com/personal/gun-control-vs-murdercrime-rates-a-statistical-look/ which finds no relevant correlation between gun control and violent crime rate. But the problem is, it's based on internal USA statistics only. The author should be asked, how such data could be relevant in any way, because we have only one country that is analyzed. What's the difference what laws are in particular states, taken there's no border control between states?

However, gun control is not 0-1 value. The ban is one thing, the control is the other. This is another factor that should be taken into account. How much arms there are per 100 people - this doesn't tell WHO has this arms, hunters, criminals or maybe collectors.

"If you want to reduce violence, you should copy the concepts from those countries, but copying single concept such as gun control is too little, the whole system should be taken into account." The problem there is that trying to do everything except gun control might not necessarily work either.
– Joe Z.Dec 28 '12 at 22:52

-1: This is a non-answer. It doesn't attempt to answer the question, but rather addresses concerns with my post, which should be handled in chat or in the comments.
– Javid JamaeJan 2 '13 at 1:06